Sex change in either direction by growth-rate advantage in the monogamous coral goby, Paragobiodon echinocephalus

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Abstract
The size-advantage model predicts the evolution of sex change if the relative reproductive success of the sexes changes with size or age. In the goby (Paragobiodon echinocephalus) the largest two fish, a male and a female matched by size, breed monogamously in each host coral. Because the female fecundity and male ability of egg care increase with body size in a similar way, no size-fecundity advantage exists. However, we found both protogyny and infrequent protandry in a natural population of this species in Okinawa. New pairs were often formed after movement between host corals and also sex change or sex differentiation of one or both members. In most new pairs males were larger than females, and females grew much faster than their mates until breeding (growth-rate advantage). The smaller member of a new pair should be the female that grows faster, because the smaller limits the reproductive success of the pair. To form such a pair, the goby changed sex according to the sex and relative size of a new mate, as a status-dependent conditional strategy. The growth-rate advantage predicts predominance of protogyny, but movement between host corals provides opportunities also for protandry.

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